1. Field of the Invention
The present invention relates to a method of manufacturing an electrode or an electrode-electrolyte membrane joint body for a fuel cell as well as to an electrode for a fuel cell. More specifically, the present invention pertains to a method of manufacturing an electrode for a fuel cell, which has a plurality of pores and includes carbon particles with a catalyst carried thereon, to a method of manufacturing an electrode-electrolyte membrane joint body for a fuel cell, which is obtained by joining such an electrode with an electrolyte membrane composed of a polymer electrolyte, and to such an electrode for a fuel cell.
2. Description of the Prior Art
In fuel cells, such as polymer electrolyte fuel cells, supplies of a gaseous fuel containing hydrogen and an oxidizing gas containing oxygen are fed respectively to two electrodes (fuel electrode and oxygen electrode) which are arranged across an electrolyte membrane, so that reactions expressed as formulae (1) and (2) given below occur to directly convert the chemical energy to electrical energy:
Anode reaction (fuel electrode):
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1) 
Cathode reaction (oxygen electrode):
2H++2exe2x88x92+(xc2xd)O2xe2x86x92H2Oxe2x80x83xe2x80x83(2) 
In order to enable these reactions to proceed in a continuous and smooth manner, the fuel electrode requires continuous supplies of water and a gaseous fuel that allow the generated hydrogen ions to be smoothly diffused into the electrolyte membrane by hydration, while the oxygen electrode requires a continuous supply of an oxidizing gas as well as quick removal of generated water. Close contact of the electrolyte membrane with both the electrodes is further required to yield a fuel cell having a small contact resistance and a high efficiency.
Several methods have been proposed to manufacture a joint body of an electrode and an electrolyte membrane which satisfies the requirements discussed above (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 6-203852). In accordance with a concrete procedure of the conventional method, a sheet-like electrode-forming member is prepared by mixing powder of a metal, such as zinc, aluminum, or chromium, or a metal salt (particle diameter: 20 xcexcm) with carbon particles having a catalyst carried thereon. After the electrode-forming member thus obtained is joined with a polymer electrolyte membrane, the electrode-forming member is soaked in a highly acidic aqueous solution, so that the metal or metal salt included in the electrode-forming member is dissolved and removed. This gives a joint body of an electrode having pores therein and an electrolyte membrane.
The conventional method of manufacturing a joint body of an electrode and an electrolyte membrane discussed above, however, has a problem, that is, metal ions dissolved in a highly acidic aqueous solution deteriorate the performance of the electrolyte membrane. The powdery metal or metal salt included in the electrode-forming member as the pore-forming agent is expected to be dissolved in a highly acidic aqueous solution and thereby removed. Even a very small quantity of metal ions remaining in the electrolyte membrane, however, combine to functional groups in the electrolyte membrane and thus drastically deteriorate the performance of the electrolyte membrane.
Another problem arising in the conventional method of manufacturing a joint body of an electrode and an electrolyte membrane is that the powdery metal or metal salt included in the electrode-forming member as the pore-forming agent is locked in the electrode and may thus not be dissolved in a highly acidic aqueous solution. When the pore-forming agent is locked inside the electrode, the resulting electrode does not have a sufficient number or size of pores. This results in insufficient permeation of a gaseous fuel or an oxidizing gas and insufficient supply and removal of water, thereby lowering the performance of the electrode.
One object of the present invention is thus to provide a method of manufacturing an electrode for a fuel cell that has sufficient gas permeability and electrical conductivity.
At least part of the above objects is realized by a first method of the present invention for manufacturing an electrode for a fuel cell. The first method of manufacturing an electrode for a fuel cell, the electrode having a plurality of pores and comprising carbon particles with a catalyst carried thereon, the first method comprising the steps of: (a) preparing an electrode-forming member comprising the carbon particles and a pore-forming agent that can be sublimed; and (b) subliming the pore-forming agent by setting the electrode-forming member under a predetermined condition.
The first method of the present invention for manufacturing an electrode for a fuel cell gives an electrode having a plurality of pores and sufficient gas permeability and electrical conductivity. The electrode-forming member is placed under a predetermined condition with a view to sublimating the pore-forming agent. This effectively prevents metals ions from remaining and lowering the performance of the electrolyte membrane and the pore-forming agent from being locked inside the electrode.
In accordance with one aspect of the present invention, the step (a) of the first method further comprises the steps of: (a1) dissolving the pore-forming agent into a solvent that allows dissolution of the pore-forming agent and dispersing the carbon particles in the solvent, so as to prepare a solution for forming a paste-like electrode; (a2) forming the solution prepared in the step (a1) into a predetermined shape of the electrode-forming member; and (a3) depositing the pore-forming agent dissolved in the solution formed into the predetermined shape in the step (a2). This structure enables the pore-forming agent to be uniformly present in the electrode-forming member, thereby allowing pores to be uniformly formed in the electrode. This further improves the performance of the electrode. The pore-forming agent generally deposits to have a needle-like or plate-like shape, which does not cause a space greater than that required for permeation of a gas in the electrode. In accordance with one aspect of this structure, the step (a3) further comprises the step of drying the solution formed into the predetermined shape, so as to make concentration of the pore-forming agent in the solution equal to or higher than its solubility, thereby allowing deposition of the pore-forming agent. In accordance with another aspect of this structure, the steps (a1) and (a2) are carried out at a predetermined temperature that allows solubility of the pore-forming agent in the solvent to be equal to or higher than a predetermined level. The solubility of the pore-forming agent in the solvent can be set to a desired level. The porosity of the electrode (that is, the ratio of pores) can thus be freely controlled by adjusting the amount of the pore-forming agent dissolved in the solvent.
The present invention is also directed to a second method of the present invention for manufacturing an electrode for a fuel cell. The second method of manufacturing an electrode for a fuel cell, the electrode having a plurality of pores and comprising carbon particles with a catalyst carried thereon, the method comprising the steps of: (a) dispersing the carbon particles in a solvent that can be freeze-dried, so as to prepare a solution for forming a paste-like electrode; (b) forming the solution prepared in the step (a) into a predetermined shape of the electrode; and (c) freeze-drying the solvent of the solution formed into the predetermined shape in the step (b) under a specified condition.
The second method of the present invention for manufacturing an electrode for a fuel cell gives an electrode having a plurality of pores and sufficient gas permeability and electrical conductivity. The plurality of pores are formed in the electrode by freeze-drying the solvent. This method accordingly does not require any specific pore-forming agent. This effectively prevents metals ions from remaining and lowering the performance of the electrolyte membrane and the pore-forming agent from being locked inside the electrode.
Other objects of the present invention are to enable an electrolyte membrane to maintain its high performance and to provide an efficient method of manufacturing a joint body of an electrode and such an electrolyte membrane.
At least part of the objects is realized by a first method of the present invention for manufacturing an electrode-electrolyte membrane joint body for a fuel cell. The first method of manufacturing an electrode-electrolyte membrane joint body for a fuel cell, the electrode-electrolyte membrane joint body being obtained by joining an electrode having a plurality of pores and comprising carbon particles with a catalyst carried thereon with an electrolyte membrane mainly composed of a polymer electrolyte, the method comprising the steps of: (a) preparing an electrode-forming member comprising the carbon particles and a pore-forming agent that sublimes under a specified condition; (b) joining the electrode-forming member prepared in the step (a) with the electrolyte membrane under a certain condition other than the specified condition to yield a joint body; and (c) subliming the pore-forming agent by setting the joint body of the electrode-forming member and the electrolyte membrane under the specified condition.
The first method of the present invention for manufacturing an electrode-electrolyte membrane joint body for a fuel cell gives a joint body of an electrolyte membrane and an electrode having a plurality of pores and sufficient gas permeability and electrical conductivity. The electrode-forming member and the electrolyte membrane are joined with each other prior to the sublimation of the pore-forming agent, so that the pores formed in the electrode are not destroyed by the joining process. The joint body of the electrode-forming member and the electrolyte membrane is placed under a predetermined condition with a view to sublimating the pore-forming agent. This effectively prevents metals ions from remaining and lowering the performance of the electrolyte membrane and the pore-forming agent from being locked inside the electrode.
In accordance with one aspect of the first method for manufacturing an electrode-electrolyte membrane joint body, the step (a) further comprises the steps of: (a1) dissolving the pore-forming agent into a solvent that allows dissolution of the pore-forming agent and dispersing the carbon particles in the solvent, so as to prepare a solution for forming a paste-like electrode; (a2) forming the solution prepared in the step (a1) into a predetermined shape of the electrode-forming member; and (a3) depositing the pore-forming agent dissolved in the solution formed into the predetermined shape in the step (a2). This structure enables the pore-forming agent to be uniformly present in the electrode-forming member, thereby allowing pores to be uniformly formed in the electrode. This further improves the performance of the electrode. The pore-forming agent generally deposits to have a needle-like or plate-like shape, which does not cause a space greater than that required for permeation of a gas in the electrode.
In accordance with one aspect of this structure, the step (a3) further comprises the step of drying the solution formed into the predetermined shape, so as to make concentration of the pore-forming agent in the solution equal to or higher than its solubility, thereby allowing deposition of the pore-forming agent. In accordance with another aspect of this structure, the steps (a1) and (a2) are carried out at a predetermined temperature that allows solubility of the pore-forming agent in the solvent to be equal to or higher than a predetermined level. The solubility of the pore-forming agent in the solvent can be set to a desired level. The porosity of the electrode can thus be freely controlled by adjusting the amount of the pore-forming agent dissolved in the solvent.
The present invention is further directed to a second method of manufacturing an electrode-electrolyte membrane joint body for a fuel cell. The second method of manufacturing an electrode-electrolyte membrane joint body for a fuel cell, the electrode-electrolyte membrane joint body being obtained by joining an electrode having a plurality of pores and comprising carbon particles with a catalyst carried thereon with an electrolyte membrane mainly composed of a polymer electrolyte, the method comprising the steps of: (a) dispersing the carbon particles in a solvent that can be freeze-dried, so as to prepare a solution for forming a paste-like electrode; (b) forming the solution prepared in the step (a) into a predetermined shape of the electrode on the electrolyte membrane; and (c) freeze-drying the solvent by setting the solution formed into the predetermined shape in the step (b) under a specified condition.
The second method of the present invention for manufacturing an electrode-electrolyte membrane joint body for a fuel cell gives a joint body of an electrolyte membrane and an electrode having a plurality of pores and sufficient gas permeability and electrical conductivity. The plurality of pores are formed in the electrode by freeze-drying the solvent. This method accordingly does not require any specific pore-forming agent. This effectively prevents metals ions from remaining and lowering the performance of the electrolyte membrane and the pore-forming agent from being locked inside the electrode.
Still another object of the present invention is to provide an electrode for a fuel cell that has sufficient gas permeability and electrical conductivity.
At least part of the above objects is realized by an electrode for a fuel cell. The electrode of the present invention comprising carbon particles with a catalyst carried thereon and a space of three-dimensional diversified structure that is made of a plurality of pores having various diameters.
The electrode of the present invention has a space of three-dimensional diversified structure that is made of a plurality of pores having various diameters. This structure improves the gas permeability and electrical conductivity.
In accordance with one aspect of the present invention, the pores formed in the electrode have diameters ranging from 0.02 to 0.3 xcexcm. The pores of small diameters further improve the gas permeability and electrical conductivity.